A typical conventional control rod driving hydraulic system for boiling water reactor will be explained first with specific reference to FIG. 1.
The control rod driving system generally has a control rod driving mechanism 1 having a hydraulic (water pressure) piston, a hydraulic unit 2 for controlling the water pressure for actuating the driving mechanism 1, and a control rod driving water pump 3 for producing the water pressure. The water pressure system includes a control rod driving water supply system A for shim operation, water supply system B for scram operation, system C for cooling the control rod driving mechanism, and a draining system D. Flow-rate regulating valves 4, pressure control valves 5, driving water headers 6, cooling water headers 7 and water drainage headers 8, all of which are known per se, are disposed at suitable portions of respective systems. Intermediate portions of the control rod driving water supplying system A are connected to valves such as control rod inserting valves 9a, 9b adapted to be opened at the time of insertion of the control rods, control rod extracting valves 10a and 10b opened at the time of extraction of the control rods, and velocity regulating valves 11a and 11b for regulating the velocity of the control rod driving water. Scram valves 12a and 12b, as well as an accumulator 13 which operates at the time of the scram operation of the reactor, are connected to an intermediate portion of the scram water supplying system B. Filters 14 are disposed at suitable portions of the control rod driving hydraulic system.
By way of reference, the control rod inserting operation of this control rod driving hydraulic system will be explained hereinunder with reference to FIG. 1.
A plurality of control rods are installed in the nuclear reactor. For inserting selected control rods into the core of the nuclear reactor, control rod inserting valves 9a and 9b corresponding to the selected control rods are opened as selector switches annexed to the control rod driving water supply system A are turned on. Therefore, the control rod driving water from the pump 3 flows through the flow-rate regulating valve 4 to the driving water header 6 and further to the selected control rod inserting valve 9a through selected branch pipes branching from the driving water header 6. The water then comes into the selected control rod driving mechanisms 1 to act on the lower surfaces of the hydraulic pistons in the mechanisms 1. As a result, each hydraulic piston produces an upward force to drive upwardly an index tube which is connected to the control rod so that the control rod is driven upwardly into the core of the reactor. Meanwhile, the water displaced by the upper surface of the hydraulic piston of the control rod driving mechanism 1 flows through the valve 9b to the water draining header 8.
For the extraction of the control rod or rods, the valves 9a and 9b are closed and the valves 10a and 10b are opened instead so that the driving water is introduced into and discharged from the driving mechanism 1 in the directions opposite to those in the rod inserting operation. Namely, the driving water is directed through the branch pipes of the driving water header 6 to the selected valves 10a and then to the selected control rod driving mechanisms 1 to act on the upper surfaces of the hydraulic pistons in these mechanisms, thereby to drive the control rod index tubes downwardly, i.e. to extract the control rods from the core of the nuclear reactor. The water displaced by the hydraulic pistons is drained through the draining header 8, past the valves 10b.
In the conventional system, the control rod driving water control unit, which is adapted to effect the control of the direction and velocity of the hydraulic pistons in the control rod driving mechanisms, are incorporated in the hydraulic units 2. As is well known, each control rod mechanism 1 has its own hydraulic unit 2. Thus the nuclear reactor is required to incorporate an impractically large number of hydraulic units. For instance, a nuclear power plant of 1,100,000 KW output necessitates a large number of, say 185, hydraulic units. Consequently, the plant as a whole has to have quite a large number of valves such as the control rod inserting valves 9a, 9b, control rod extracting valves 10a, 10b and velocity regulating valves 11a, 11b. Obviously, the increased number of valves employed correspondingly increases the chance or frequency of troubles occurring in the hydraulic unit. The large number of valves incorporated in each of the hydraulic units 2 for each of a plurality of control rod driving mechanisms inevitably prolongs the length of time for the periodical inspection of the plant. This is quite unfavourable from the view point of reduction of the rate of dosage on the workers.